The present invention relates to a light-wave rangefinder using a pulse method, and more particularly to a light-wave rangefinder using a pulse method, which detects an optical noise generated as internal reflection of a main body of the light-wave rangefinder to store the optical noise as optical noise data, and which uses the optical noise data to correct distance measuring data so that a measurement error is reduced.
The light-wave rangefinder using a pulse method developed in recent years is capable of non-prism distance measuring. To be more specific, the light-wave rangefinder can measure a distance using light reflected from a target to be measured itself without using a reflection device, such as a prism, as a target to be measured. The non-prism distance measuring includes the following: receiving feeble reflected light, which is reflected by the surface of a target to be measured, to perform distance measuring; and emitting pulsed light with high peak power, and receiving reflected light, which is reflected by the surface of a target to be measured itself, to perform distance measuring.
The received reflected light is converted into an electric signal, and then various kinds of processing for calculation of a distance is performed according to the electric signal. Therefore, the reflected light is received and converted into an electric signal in various methods that are considered so that a stable signal can always be obtained regardless of disturbance.
Here, one of methods for obtaining a stable signal will be described with reference to FIG. 9. Pulsed light (a) emitted from a light-wave rangefinder is reflected by a target to be measured, and is delayed by a period of time in response to a distance from the light-wave rangefinder to the target to be measured before the pulsed light is received as reflected pulsed light (b) by a light receiving element. A tuned amplifier changes a waveform of the reflected pulsed light (b), which has been received, to a damped oscillation waveform (c). This damped oscillation waveform can be obtained by setting Q of the tuned amplifier properly. This method has an advantage mentioned below. Even if a peak value of the reflected pulsed light (b) changes owing to fluctuations in the air, so long as a position of its center of gravity remains unchanged, a zero-crossing point of the damped oscillation waveform (c) does not change. Therefore, using this zero-crossing point as reference timing of distance measuring operation enables accurate measurement.
By the way, the longest distance to be measured in the non-prism distance measuring by a light-wave rangefinder using a pulse method tends to become greater year-by-year.
Examples of methods for increasing a measurable distance of the non-prism distance measuring include emitting light with higher peak power; and increasing light-receiving sensitivity to improve efficiency in light receiving of feeble reflected light. However, this poses a problem that feeble optical noise, which is reflected by the surface of an optical member used in the light-wave rangefinder, is also received simultaneously. This optical noise causes a measurement error when measuring a short distance. In particular, the influence is large when the non-prism distance measuring is used, which is a serious problem.
Next, the influence of an optical noise exerted upon distance measuring operation will be described with reference to FIG. 10. As shown in FIG. 10(a), if reflected light is received immediately after an optical noise is received, a damped oscillation waveform which is generated from the optical noise and the reflected light has a shape as shown in FIG. 10(b). Accordingly, as shown in FIG. 10(c), the damped oscillation waveform, which is actually observed, becomes a waveform into which a damped oscillation waveform of the optical noise and a damped oscillation waveform of the reflected light are combined. As a result, the following problem arises: the influence of the optical noise causes a slight phase shift of a zero-crossing point of this combined damped oscillation waveform, which produces an error in measured distance.
Therefore, conventionally, there were the following serious problems: adjustment for eliminating the influence of the optical noise must be performed at the time of factory shipment, which requires much labor and hinders an improvement in productivity. The prior art shown in FIG. 11 includes a crystal oscillator 100, a first frequency divider 110, a synthesizer 120, a second frequency divider 130, a third frequency divider 140, a luminous element 1, a luminous element driver 150, a light receiving element 71, an amplifier 160, a zero-crossing detecting circuit 165, a waveform shaping circuit 170, a counter 180, a peak-hold circuit 190, a level judging circuit 200, a band-pass filter 210, a sample hold (S/H) 220, an arithmetic processing circuit 1000, and an optical-noise nonvolatile memory 620.
The prior art is so devised that adjustment for eliminating the influence of an optical noise is performed at the time of factory shipment, and then data is stored in the optical-noise nonvolatile memory 620. In this connection, since the other configurations will be described in embodiments mentioned below, the description will be omitted here.
According to one aspect of the present invention there is provided a light-wave rangefinder using a pulse method, which can reduce a measurement error. In the light-wave rangefinder, an optical noise sampling unit samples an optical noise produced in the rangefinder; an optical-noise storage unit stores sampling data of the optical-noise sampling unit; and an arithmetic processing means reduces a measurement error, which is caused by an optical noise, on the basis of the sampling data of the optical-noise storage unit, and thereby a distance, a measurement error of which is reduced, can be calculated.